Vacuum: The Base for New Technologies

The technology of vacuum plays a pivotal role in making these wide range of applications available.

Essentially, generating a vacuum means sucking out all the air from a particular area. The higher number of air molecules you can pull out, higher is the vacuum obtained. Vacuum technology is the term applied to all processes and physical measurement carried out below-normal atmospheric pressures. The units of vacuum are mbar and torr.

1. Low
2. Medium
3. High
4. Ultrahigh

But the question arises, why should I remove the atmospheric molecules to perform one of the above-mentioned applications?

Some of the reasons as to why a process or physical measurement is generally performed under vacuum are:

• To remove the constituents of the atmosphere that could cause a physical or chemical reaction during the process (e.g., vacuum melting of reactive metals such as titanium).

• To disturb an equilibrium condition that exists at normal room conditions, such as the removal of occluded or dissolved gas or volatile liquid from the bulk of material (e.g., degassing of oils, freeze-drying) or desorption of gas from surfaces (e.g., the cleanup of microwave tubes and linear accelerators during manufacture).

• To extend the distance that a particle must travel before it collides with another, thereby helping the particles in a process to move without collision between source and target (e.g., in vacuum coating, particle accelerators, television picture tubes).

• To reduce the number of molecular impacts per second, thus reducing chances of contamination of surfaces prepared in vacuum (e.g., in clean-surface studies and preparation of pure, thin films).

For any vacuum process, the limiting parameter for the maximum permissible pressure can be defined by:

1. The number of molecules per unit volume (reasons 1 and 2),
2. The mean free path (reason 3),
3. Or the time required to form a monolayer (reason 4).

At room temperature and normal atmospheric pressure, one cubic foot (0.03 cubic meters) of air contains approximately 7x10 raised to(23) molecules moving in random directions and at speeds of around 1,000 miles per hour. The momentum exchange imparted to the walls is equal to a force of 14.7 pounds for every square inch of wall area.

This atmospheric pressure can be expressed in a number of different units, but until recently, it was commonly expressed in terms of the weight of a column of mercury of unit cross section and 760 millimeters (mm) high.

Thus, one standard atmosphere equals 760 mm Hg, but to avoid the anomaly of equating apparently different units, a term, torr, has been postulated.

So one standard atmosphere = 760 torr (1 torr =1 mm Hg).

This term was replaced in 1971 by SI unit defined as the newton per square meter (N/m raised to (2)), and called the pascal (one pascal=7.5x10 raised to (-3) torr).

The first major use of this technology in industry occurred about 1900 in the manufacture of electric light bulbs. Other devices requiring vacuum for their operation followed, such as various types of electron tube.

Furthermore, it was discovered that certain processes carried out in a vacuum achieved either superior results or ends actually unattainable under normal atmospheric conditions.

Such developments included the "blooming" of lens surfaces to increase the light transmission, the preparation of blood plasma for blood banks, and the production of reactive metals such as titanium.

The advent of nuclear energy in the 1950s provided momentum for development of vacuum equipment on a large-scale. Increasing applications were steadily discovered, as in space simulation and microelectronics.

Industrial applications range from mechanical handling (such as the manipulation of heavy and light items by suction pads) to the deposition of integrated electronic circuits on silicon chips. Obviously, vacuum requirements are as widely varied as the particular processes using vacuums.

In the rough vacuum range from about one torr to near atmosphere, typical applications are:

1. Mechanical handling
2. Vacuum packing and forming
3. Gas sampling
4. Filtration
5. Degassing of oils
6. Concentration of aqueous solutions
7. Impregnation of electrical components
8. Distillation
9. Steel stream degassing

At lower pressures down to about 10 raised to (-4) torr, many metallurgical processes such as melting, casting, sintering, heat treatment, and brazing can derive benefit. Chemical processes such as vacuum distillation and freeze-drying also need this range of vacuum.

Freeze-drying is used extensively in the pharmaceutical industry to prepare vaccines and antibiotics and to store skin and blood plasma. The food industry freeze-dries coffee mainly, although most foods can be stored without refrigeration after freeze-drying, and the technique is receiving widespread acceptance.

The pressure range down to about 10 raised to (-6) torr is used for cryogenic (low-temperature) and electrical insulation. It is used in the production of lamps; television picture tubes, X-ray tubes; decorative, optical, and electrical thin-film coatings; and mass spectrometer leak detectors.

In thin-film coating, a metal or compound is evaporated under high vacuum from a source onto a base material or substrate. The base material is generally plastic for decorative coatings; glass for optical coatings; and glass ceramic, or silica for electrical coatings.

Thickness of the film can vary from about 1/4 wavelength of visible light to 0.001 inches or more. In the optical field, antireflection coatings are deposited on lenses for cameras, telescopes, eyeglasses, and other optical devices, considerably reducing the amount of light reflected by the lenses and thus giving a brighter transmitted image.

To achieve vacuum, high enough for thin-film coating and for other industrial uses requiring pressures down to 10 raised to (-6) torr, a pumping system consisting of an oil-sealed rotary pump and a diffusion pump is used.

The oil-sealed rotary pump "roughs" the chamber down to a pressure of about 0.1 torr, after which the roughing valve is closed. The fore valve and high-vacuum baffle valve are then opened so that the chamber is evacuated by the diffusion pump and rotary pump in series.

Almost every research laboratory uses vacuum directly in its experiments or employs equipment that depends on it for its operation. The lowest pressures are obtained in the research laboratories, where equipment is generally similar to, but smaller than that used by industry.

Typical of the research equipment using vacuum down to about 10 raised to (-6) torr are the electron microscope, analytical mass spectrometer, plasma chambers, particle accelerator, and large space simulation equipment. Particle accelerators range from small Van De Graaff machines to large proton synchrotrons.

In space simulation, large units that simulate space around a complete vehicle require a vacuum of 10 raised to (-6) torr or below. Such vessels incorporate a complete shroud at liquid nitrogen temperature and a port through which high-intensity light can be beamed to simulate the sun's radiation.

In the pressure region down to and below 10 raised to(-9) torr, research applications include electrical insulation, thermonuclear energy conversion experiments, microwave tubes, field ion microscopes, field emission microscopes, storage rings for particle accelerators, specialized space simulator experiments, and clean-surface studies.

In many experiments it is not only necessary to reach such pressures of 10-9 torr but to reduce the hydrocarbons in the residual gases to an absolute minimum. Even small traces of hydrocarbons can render the results unreliable.

To achieve a vacuum of this order, the vessel and the equipment inside must be cleared of residual gas (degassed) to the greatest extent possible.

A common solution is to bake the whole apparatus for a number of hours at about 350 degrees C while maintaining a vacuum in the 10 raised to (-5) torr region. To eliminate hydrocarbons, the unit is pumped down to about 10 raised to(-3) torr using sorption pumps; and from there, sputter ion pumps and titanium sublimation pumps complete the task down to 10 raised to (-9) torr or below.

Every technological development of today relies heavily on the development of vacuum related technology that would generate good vacuum. Rotary pumps, Sorption pumps, Diffusion pumps, Turbomolecular pumps, ion pumps, getter pumps... and the list goes on, all are elements of an evacuation system typically used in industries and research.

Of course, faster evacuating and less power consuming pumps are in demand for industries, just as faster circuit speed and lower power consumption are required for very large-scale integration of electronic circuits.